In many applications, it is necessary to measure the position of a rotary shaft of a rotational device. However, rotational devices are often complex and have parts that are difficult to access. Furthermore, rotational devices are often integrated into industrial processes where the cost of stopping the process to repair the rotational device often far exceeds the cost of the rotational device. Rotary valves, for example, often are critical to industrial processes and repair of some parts of the valves require shutting down the process. A need exists to precisely identify the position of a rotary shaft and objects driven by the rotary shaft, such as a valve stem. A need also exists to identify any wearing parts in a rotational device, such as a valve, so that preventative maintenance can be performed at scheduled shutdowns, or so that the rotational device can be operated in such a way as to keep the device operational until the next scheduled shutdown. A need exists for a device capable of both determining the position of a rotary shaft as well as identifying the severity and location of problems within the rotational device to which the rotary shaft is connected.
One approach to diagnosing rotating devices has employed frequency analysis. Cyclic data may be analyzed with a Fourier Transform (FT) algorithm to transform the data from a time domain to a frequency domain. One attempt to apply FT to motor-operated valves involved measuring the current flowing to the motor, applying FT to the motor data, and then using peaks in the frequency spectrum to diagnose problems in the drive train of the valve actuator. However, this approach does not measure the rotational speed of a shaft nor does it determine the position of a rotary shaft. A motor current-measuring device also does not integrate into a device capable of determining the position of a rotary shaft.
One approach to measuring the position of a rotary member involves a rotary encoder. Rotary encoders include incremental and absolute encoders. Incremental encoders are used to measure the rotational change of a shaft. A basic incremental encoder includes a disk with a large number of radial painted lines. A photodiode or other sensor generates an electrical pulse whenever a painted line is sensed. A computer, or other processor, tracks the pulses to determine the position of the disk and, in turn, the position of the shaft to which the disk is attached. With incremental encoders, if power is lost to the computer, the position information is lost when power is restored. Previous incremental encoders for valve actuators have included a speed sensor, but the speed sensor and resulting data have not been used for frequency analysis.
Absolute encoders do not require a power supply to maintain position information. Absolute encoders produce a unique digital code for each distinct angle of a rotary shaft. Absolute encoders can be a single wheel with a complex pattern machined into the wheel. The single wheel is attached to the shaft in question and numerous distinct angular positions can be identified by the patterns on the wheel. However, such wheels are only useful where a shaft will undergo only a single rotation.
Another version of the absolute encoder utilizes multiple wheels with concentric rings on each of the wheels, where each ring provides one bit of position data. The multi-wheel version allows the measured shaft to undergo numerous rotations and still track the position and number of rotations of the shaft. The presence of more wheels allows tracking of more shaft rotation or determination of more positions for a single rotation. However, multi-wheel absolute encoders are often delicate and less reliable. It would be desirable to have a multi-wheel absolute encoder that is reliable and operable to generate speed data for use in frequency analysis.
One attempt to solve this problem utilizes either six or seven wheels. Each wheel provides three bits of data. However, only two bits of Gray code are generated as position data via v-bit processing. This increases the reliability of the absolute encoder. However, duplicate sensors are not used. Additionally, a speed sensor is not integrated into the absolute encoder and speed data is not generated for use in frequency analysis.